1. Field of the Invention
The present invention relates to an electric power steering apparatus that directly applies the power of an electric motor to a steering system as a steering assist torque to reduce the steering torque required from the driver.
2. Description of the Related Art
An electric power steering apparatus has been proposed by the applicant in U.S. Ser No. 08/353300 filed on Dec. 6, 1994 wherein the steering system includes an electric motor and wherein a power supplied by the electric motor is controlled by a controller to generate a steering assist torque in order to reduce the steering torque required from the driver.
Conventional electric power steering apparatuses comprise a steering torque detection means for detecting the steering torque of a steering system and a steering rotating speed detection means for detecting the rotating speed of the steering system, and causes a controller to provide an electric motor control signal (for example, a target current value) based on the detected steering torque and rotating speed in order to drive the electric motor via an electric motor drive means, thereby providing a steering assist torque corresponding to the steering torque effected by the driver.
Furthermore, conventional power steering apparatuses generate an electric motor control signal suitable to the travel state of the vehicle based on a steering rotating speed signal from the steering rotating speed detection means and a car velocity signal from the car velocity detection means to, for example, apply a large amount of damping to the steering system in order to stabilize the behavior of the vehicle during fast travel, while reducing the amount of damping applied to the steering system during slow travel to improve the returning of the steering wheel, thereby enabling the generation of damping according to the velocity of the vehicle.
A known electric power steering apparatus that eliminates a sense of delayed wheel-turning due to a delay in the response from the steering system at the beginning of steering by controlling the assist torque based on the sum of a value proportional to a steering torque signal and a differential value of the same signal is also disclosed in Japanese Laid Open 63-19386.
Although conventional power steering apparatuses controls the damping characteristics of the steering system so that the amount of damping will be reduced during slow travel to increase the steering assist torque while the amount of damping will be increased during fast travel to reduce the steering assist torque, thereby varying the amount of damping according to the car velocity to improve the steering feeling of the vehicle, they have the following problems.
If, for example, a quick wheel operation is required during fast travel when the amount of damping is increasing and the driver attempts to perform a wheel operation with a high steering rotating speed, a large steering torque is required due to the small set value of the steering assist torque, preventing a desired steering operation.
In addition, since damping also occurs near the neutral position of the steering wheel, a large steering torque is required for a steering operation with a quickly varying steering speed, resulting in a degraded steering feeling.